1. Field of the Invention
This invention relates to alkaline batteries containing divalent silver oxide cathode material. More particularly, it relates to additives for such batteries.
2. Prior Art
In the packaged power industry, there is an ever increasing emphasis upon the development of high capacity, small volume electric cells and batteries. The following is a comparison of some positive active materials currently used in commercially available cells:
______________________________________ Active EMF vs. ZN in Capacity Material Alkaline Electrolyte ma-hr/g a-hr/cc ______________________________________ HgO 1.35v. 248 2.76 Ag.sub.2 O 1.60v. 232 1.67 AgO 1.82v. 432 3.22 ______________________________________
The above comparisons clearly indicate that divalent silver oxide (AgO) has the highest capacity per gram or cubic centimeter and also the highest EMF. With the advancement of semi-conductor technology and the increasing use of semi-conductors in electronic devices, there is greater than ever need for high capacity, small volume packaged power sources having high voltages.
As shown by the preceding table, divalent silver oxide is an excellent high capacity positive active material having a relatively high EMF, but unfortunately, it has limited use as a battery active material because of its lack of stability when in contact with aqueous alkaline solutions. It is well-known that divalent silver oxide evolves oxygen when in contact with aqueous alkaline solutions, for example sodium hydroxide and potassium hydroxide, as represented by the following reaction: EQU 2AgO -- Ag.sub.2 O + 1/2O.sub. 2
as a result of this instability, alkaline batteries employing a divalent silver oxide positive material suffer a loss of capacity on activated stand because of the conversion of the divalent silver oxide to monovalent silver oxide. In addition, the gassing of the divalent silver oxide precludes its use in sealed cells and batteries because of the hazard of pressure build-up and possible explosion of the sealed cell. The formation of gas bubbles within the sealed cell also increases the impedance of the cell.
The properties of divalent silver oxide active material have been studied by many persons skilled in the battery art, and there are two articles relating to additives for silver oxide which have been published and which are particularly pertinent to this invention. "Electrode Phenomena of Silver-Silver Oxide System in Alkaline Batteries" by Shiro Yoshizawa and Zenichro Takehara published in the Journal of the Electrochemical Society of Japan, Volume 31, Number 3, pages 91-104 (1963) reports the effect of various metallic additives on the oxidation of silver electrodes. Among the additives studied by the Japanese, was gold which was reported to increase the rate of formation of divalent silver oxide during the electrochemical formation of silver electrodes, i.e., oxidation of silver. Another article entitled "The Electric Resistivity of Silver Oxide" by Aladar Tvarusko published in the Journal of the Electrochemical Society, Volume 115, Number 11, pages 1105- 1110 (November, 1968) reported on various metallic additives and their effect on the electric resistivity of divalent silver oxide. The article reports that mercury added during the preparation of silver oxide decreased the electric resistivity of silver oxide.
The patent literature also contains publications disclosing additives for alkaline batteries employing silver positive electrodes. U.S. Pat. No. 3,617,384 issued to Kamai et al on Nov. 2, 1971 discloses a secondary zinc alkaline cell in which gold or silver, alloys thereof, oxides and hydroxides may be added to the zinc anode. U.S. Pat. No. 3,650,832 issued to Aladar Tvarusko on March 21, 1972 discloses certain additives for divalent silver oxide selected from mercury, selenium, tellurium and combinations of mercury with tin or lead. Japanese Patent Application No. 48- 1929 was open to public inspection on Jan. 22, 1973 discloses adding gold hydroxide into the electrolyte of an alkaline silver oxide cell. U.S. Pat. No. 3,853,623 issued on Dec. 10, 1974 to Stuart M. Davis discloses gold ion additive for divalent silver oxide. U.S. Pat. No. 3,935,026 issued on Jan. 27, 1976 to Paul L. Howard discloses a cathode material which is a mixture of divalent silver oxide and sulfur and a cathode material which is a mixture of divalent silver oxide and silver sulfide. Great Britain Patent No. 1,065,059 published Apr. 12, 1967 discloses the addition of cadmium oxide, magnesium oxide, or aluminum oxide to a silver electrode.